In the area of commercial and industrial printing (e.g. printing of promotion material, product packages or newspapers), offset printing is the prevalent printing technique. Conventional offset printing machines have to compete increasingly with new electrophotographic printing systems which are capable of producing commercial and industrial printing products with similar high quality. These electrophotographic printing systems use liquid toner or liquid inks in order to attain a high spatial resolution.
It has been proposed to compress images to be printed in order to decrease bandwidth load while transmitting images from a workstation (where an image might have been created or processed) to a printing system and to economize memory and computing power of printing systems. U.S. Pat. No. 7,145,696 B2 to Silverbrook describes how printing data can be compressed by using different compression methods for contone layers and two-color layers. U.S. Pat. No. 2006/0181720 to Kakutani proposes differentiation between edges, which are not to be compressed, and other areas within an image, which are to be compressed. Another approach is disclosed in U.S. Pat. No. 5,664,406, where image blocks of 4×4 pixels are encoded using 32 bits. Depending on the identified color contrast within a block, an adequate compression algorithm for this block is chosen. If, for example, the block contains only one uniform color, all 32 bits can be used for color representation, whereas if a block contains a larger quantity of colors, a major part of the 32 bits is used for representation of spatial resolution and therefore only few bits are left for color representation. The result is an image that is subdivided into several pieces and encoded with various encoding techniques. A comparable approach is shown in U.S. Pat. No. 6,091,511 to Ben Dror et al., wherein the gray-level contrast within a block of pixels is determined and either a high spatial resolution with a narrow gray-level range or a low spatial resolution with a wide gray-level range is selected.